The invention relates to conveyor belt systems, and more particularly to a low tension system wherein a plastic conveyor belt is used, in such a way as to eliminate most wear problems.
Plastic conveyor belts have been in increasing use, particularly in the food industry where dwell time in an oven, a freezer or other food treating environment is needed, for a maximum amount of product and with minimum space requirement. These belts are also used in the electronics and computer industries. See, for example, U.S. Pat. No. 4,742,907 directed to a plastic conveyor belt formed of a series of modules, assigned to the same assignee as the present invention. Such a plastic conveyor belt is designed to travel in straight paths and around curves, and may make both left and right curves. The belt collapses on the inside of curves, having rods connecting adjacent belt segments through slotted holes.
Some plastic conveyor belts have had end members or end plates which are thickened and have recesses at their outer surfaces, for recessing the head of a rod so that the rod head does not protrude out from the end member or end plate. This prevents snagging of the heads on components of the belt driving or guidance system.
In low tension spiral conveyor belt systems (such as Ashworth Low Tension Spiral System) a belt, generally of metal, travels in a helical path around a rotating driving tower or cage or drum. The belt passes over a driven sprocket at a take-up drive location separate from the driving tower. In passing around and in contact with the driving tower, the belt is friction driven, assisted in its movement generally throughout the length of the belt's path on the tower, greatly reducing tension in the belt as compared to tension which would occur if the belt were only driven from one point. The belt is actually driven at a great number of points of contact with the cage or drum.
In such low tension systems, the edges of the belts have been engaged generally by bars or vertical members or rods of the driving cage or tower. The edges of the belts have generally been irregular, with end structure of the component belt modules exposed and irregular and not smooth. This is often due to exposed and protruding heads of transverse module connecting rods, and results in snagging of the rod heads against the driving cage and in excessive wear of the rod heads or other protruding components or surfaces of the belt as well as of the cage members. The problem is particularly accentuated by the fact that the tower or cage must be driven at a speed somewhat faster than that of the belt in an overdrive condition, inasmuch as the belt is moving both horizontally and vertically along the cage. A slipping condition therefore exists between the edge surfaces of the belt and the external surfaces of the driving tower or cage. This condition must exist whether the belt starts from the bottom and ends up at the top or starts from the top and ends up at the bottom in relation to the moving cage. (The cage is actually driving the belt supported by spiral support strips around the drum or the cage, up or down, as shown in the drawings and discussed further below.)
Previous to the present invention, there has been no suggestion for solving this problem by eliminating snagging, tearing and catching and thereby improving the performance, wearability and life of a low tension conveyor belt system utilizing a plastic conveyor belt, as in the present invention described below.